High intensity arc discharge lamps, such as metal halide arc discharge lamps, include a light emitting arc discharge tube hermetically sealed within a light transmissive, vitreous lamp envelope. Electrical energy is coupled through a metal lamp base to the arc tube. Metal halide arc tubes provide excellent color, long life and high efficiency. The arc tube is generally cylindrical, having a longitudinal axis and made of fused quartz hermetically sealed at both ends over a pair of opposing electrodes by a pinch or press seal. It contains an ionizable fill sealed within for forming a visible light-emitting arc when the electrodes are energized. The fill contains mercury and a halide of sodium and one or more metals such as scandium, thorium, thallium, praseodymium, neodymium, cesium, cerium, etc., and an inert starting gas such as argon. The arc tube can also be made of a light transmissive ceramic, such as polycrystalline or single crystal alumina as disclosed in U.S. Pat. No. 5,424,609. Metal halide arc discharge lamps frequently incorporate a shroud which provides both performance and safety improvements. The shroud comprises a cylindrical, light transmissive member, such as fused quartz, which is able to withstand the high operating temperatures of the lamp and, at the same time, serve as a containment means to protect the environment external to the lamp from shards of the arc tube in the rare event that one should burst. The arc tube and the shroud are coaxially mounted within the lamp envelope, with the arc tube concentrically positioned within the shroud. Thus, a constant and uniform distance is provided between the inner wall of the shroud and the outer wall of the arc tube.
Arc tube shrouds are disclosed as being open at both ends, open at one end and closed at the other end by a domed configuration, and also capped at both ends by perforated metal caps. The shrouds are also heat conserving means which reduce arc tube heat loss by absorbing heat emitted by the arc tube and reradiating a portion of the absorbed heat back to the tube. This results in a more even temperature distribution over the surface of the arc tube than if the shroud were not present. Such shrouds and methods for mounting them around the arc tube are disclosed in, for example, U.S. Pat. Nos. 4,499,396; 4,580,989; 5,075,588 and 5,252,885, all assigned to the assignee of the present invention. However, even with arc discharge lamps incorporating a shroud, the upper portion of the arc tube is hotter than the lower portion due to gas convection within the tube. Thus, when operated in a vertical position, the bottom of the arc tube is cooler than the top, and when operated in a horizontal position, the bottom portion or side of the arc tube is cooler than the upper portion. Surrounding the arc tube with a cylindrical shroud of the prior art has provided some benefits in reducing the temperature differential between the coldest and hottest portions of the arc tube to a value less than it would be without the shroud as disclosed, for example, in U.S. Pat. No. 4,859,899. High temperature, heat reflecting coatings have also been used on arc tubes to reduce the temperature differential between the hot and cold spots, but the results haven't been as good as shrouded arc tubes. Also, the use of coatings requires significantly greater amounts of expensive getters within the outer envelope of the lamp to getter gasses given off by the coatings at the extremely hot operating temperatures (e.g., 800.degree. C.) of the arc tube.